A number of applications exist for monitoring temperatures at different locations in a system and then either displaying the various monitored temperatures or utilizing these temperatures to control the operation of the system in a particular manner dependent upon the relationship of the temperatures being monitored. Typical examples are in the field of monitoring the temperatures in a plurality of zones in an aircraft engine, monitoring the operating temperatures of various cylinders of an internal combustion engine or diesel engine, monitoring temperatures inside and outside a building, and the like.
In their simplest form, temerature monitoring devices, in the general category discussed above, exist in the form of the well known indoor/outdoor thermometers. Generally, such thermometers employ dual analog displays, each activated by a different sensor, one located inside the room in which the display is located and the other located outside the room. Such a simple device actually is nothing more than two independent temperature sensor/display units, with the displays conveniently located at a single place.
More sophisticated temperature monitoring systems exist for providing temperature displays indicative of the operating conditions of engines, for example of the type used in aircraft. One such system is disclosed in the patent to Pratt, U.S. Pat. No. 4,114,442, issued Sept. 19, 1978. The Pratt patent is used to continuously and automatically monitor and alternately display exhaust gas temperature, cylinder head temperature, and turbine inlet gas temperature data to the pilot of an aircraft. In the system of the Pratt patent, temperature sensors are located in different temperature monitoring zones within the aircraft engine. These sensors comprise thermocouples operating at the extremely high temperatures encountered in an aircraft engine (on the order to 1,000.degree. F. or higher) and generate voltages which then are applied to the monitoring system to produce output indicia of the temperature of the various zones in which the sensors are located. The temperature monitoring system of the Pratt patent also includes a circuit for automatically and sequentially scanning the various sensor output signals; so that they are displayed to the pilot in a predetermined pattern. Since, in many cases, it is possible to detect the malfunctioning of an aircraft engine when its temperature exceeds some predetermined maximum at some point in the engine, the Pratt system includes an alarm which is activated at any time the threshold temperature detected by the sensor connected with the alarm exceeds a temperature which previously has been determined as a maximum operating temperature for that zone of the engine. In addition, the system also actuates an alarm whenever the difference between the hottest and coldest exhaust gas temperatures from different engines exceeds a preestablished value.
The system of the Pratt patent, however, must employ thermocouples because of the high temperatures encountered and no negative temperatures are sensed or displayed by the system. Thermocouples have serious corrosion problems; and because they are voltage generators, generating voltages in the millivolt range, the sensor must be located as near as possible to the display panel. This is necessary to avoid the introduction of substantial errors in the data displayed as a result of resistence losses in the interconnections between the thermocouple and the display panel.
Other patents which are generally related to the field of the present invention include the patents to Smith, U.S. Pat. No. 2,445,156, issued July 13, 1948; Hoffman, U.S. Pat. No. 3,077,775, issued Feb. 19, 1963; and, Podl, U.S. Pat. No. 4,122,720, issued Oct. 31, 1978.
The Smith patent is directed to a system for measuring temperature differences between the cooling water and the exhaust temperature for the different cylinders of a diesel engine. As in the Pratt patent, thermocouples are used as the temperature sensors to generate voltages indicative of the temperatures in the various zones where the thermocouples are located. Smith recognized the problem inherent with the use of thermocouples as a result of the resistence of the wires connecting the sensors with the reading panel of the instrument and attempted to overcome the difficulty by making all of the lead wires from all of the thermocouples exactly the same length. This requires a very precise installation; and, obviously, if repairs subsequent to the initial installation later are made, it is necessary to observe this critical restriction or erroneous information will be displayed by the system thereby substantially reducing its effectiveness or actually making it unreliable for its intended purpose. Once again, the Smith patent system is useful only in relatively high temperature application.
The Podl patent is similar to the Smith patent and is directed to a temperature monitoring system for a diesel engine which has sequential monitoring of the sensors associated with different cylinders by means of a multiplexing system. The signals from the temperature sensors (again thermocouples with all of their inherent disadvantages) are connected to comparator circuits, and an alarm is sounded if the temperature sensed is outside of some preestablished limit determined by a reference signal supplied to the comparator circuitry. Generally, the alarm is activated in response to a high temperature condition over some preestablished "safe" threshold. The system also provides an alarm for a deviation in temperature, either above or below some preestablished norm, and also for a temperature which is more than some preestablished amount below a preestablished set point.
The Hoffman patent is of general interest for a system utilizing thermocouples or other input devices for sequentially scanning various inputs and connecting them to a common output circuit. For temperature inputs, Hoffman, as with the above discussed patents, employs voltage generating thermocouples; so that Hoffman also is subject to the disadvantages of such systems and the limitations which thermocouple sensors produce in any system with which they are used.
Other multiple temperature sensing systems which are used for various applications in the prior art include the systems disclosed in the patents to Vidts, U.S. Pat. No. 3,404,570, issued May 25, 1966; Tsoras, U.S. Pat. No. 3,688,295, issued Aug. 29, 1972; Sarafin, U.S. Pat. No. 2,900,820, issued Aug. 25, 1959; and, Nitschke, U.S. Pat. No. 4,130,019, issued Dec. 19, 1978.
The Vidts patent discloses a system for sensing multiple temperatures at separated sensing zones and also displaying the temperature difference between different zones. The Sarafin patent is similar in its application to the indoor/outdoor thermometers discussed previously, but is directed to a differential oral thermometer for use with medical patients. The Tsoras et al patent discloses yet another plural thermocouple temperature monitoring system in which the multiple sensors are connected individually to different comparators. The outputs of the various comparator circuits then are connected in common to operate an alarm if any one of the input temperatures which are sensed is in excess of some maximum established by the reference signal applied to the various comparators. The system disclosed in the Nitschke patent also is a thermocouple system (with all of the disadvantages of such sensors) which has a number of outputs applied through a multiplexer to a computer for further processing. In the computer, operations are made to nullify offset errors of various portions of the circuit electronics and to correct for the inherent errors introduced into such a monitoring system by the cold junction of the thermocouples used.
None of the foregoing systems permit the alternative sensing and displaying of both positive and negative temperatures by any one or more of the temperature sensor/display devices utilized. In addition, the comparative temperature systems which are known in the prior art generally employ thermocouple sensors; so that they are limited to operating in high temperature environments and cannot mix positive and negative temperature sensing for utilization in the same system.
It is desirable to provide a system capable of operating at relatively low temperatures, which is not subject to the inherent limitations of thermocouple temperature sensing systems, and which further is capable of displaying positive and negative temperatures. In addition, the system should be capable of providing output difference signals indicative of the amount of temperature difference between two sensors and also indicative of which of those sensors is at a higher temperature than the other.